EventStream deadLetters doc has been improved (#6662)

* Documentation of EventStream unhandled messages notification has been improved (#5334).

* Fix after review.

* Fix after review.

* Update event-bus.md

---------

Co-authored-by: Aaron Stannard <[email protected]>

Author: F0b0s

docs/articles/utilities/event-bus.md

@@ -4,6 +4,14 @@ title: Event Bus
 ---
 # EventBus
 
+EventBus provides several types of custom messages to subscribe to:
+
+1. `DeadLetter` - messages that are not delivered to actor
+2. `UnhandledMessage` - messages which actor receives and doesn't understand
+3. `SuppressedDeadLetter` - similar to DeadLetter with the slight twist of NOT being logged by the default dead letters listener
+4. `Dropped` - messages dropped due to overfull queues or routers with no routees
+5. `AllDeadLetters` - shortcut for all types of unhandled messages
+
 ## Subscribing to Dead Letter Messages
 
 The following example demonstrates the capturing of dead letter messages generated from a stopped actor.  The dedicated actor will output the message, sender and recipient of the captured dead letter to the console.
@@ -58,6 +66,85 @@ sample capture
 DeadLetter captured: another message, sender: [akka://MySystem/deadLetters], recipient: [akka://MySystem/user/ExpendableActor#1469246785]
 ```
 
+## Subscribing to Unhandled Messages
+
+The following example demonstrates the capturing of unhandled messages. The dedicated actor will output captured unhandled message to the console.
+
+```csharp
+public class DumbActor : ReceiveActor {  }
+
+public class UnhandledMessagesMonitorActor : ReceiveActor
+{
+    public UnhandledMessagesMonitorActor()
+    {
+        Receive<UnhandledMessage>(Console.WriteLine);
+    }
+}
+
+using (var system = ActorSystem.Create("MySystem"))
+{
+    var dumbActor = system.ActorOf<DumbActor>();
+    var monitorActor = system.ActorOf<UnhandledMessagesMonitorActor>();
+    
+    // Subscribe to messages of type UnhandledMessage
+    system.EventStream.Subscribe(monitorActor, typeof(UnhandledMessage));
+    
+    // try sending a message to actor which it doesn't understand
+    dumbActor.Tell("Hello");
+    
+    Console.ReadLine();
+}
+```
+
+sample capture
+
+```string
+DeadLetter from [akka://MySystem/deadLetters] to [akka://MySystem/user/$a#965879198]: <Hello>
+```
+
+## Subscribing to AllDeadLetters Messages
+
+The following example demonstrates the capturing of all unhandled messages types. The dedicated actor will output captured unhandled messages to the console.
+
+```csharp
+public class DumbActor : ReceiveActor {  }
+
+public class AllDeadLettersMessagesMonitorActor : ReceiveActor
+{
+    public AllDeadLettersMessagesMonitorActor()
+    {
+        Receive<AllDeadLetters>(Console.WriteLine);
+    }
+}
+
+using (var system = ActorSystem.Create("MySystem"))
+{
+    var dumbActor = system.ActorOf<DumbActor>();
+    var monitorActor = system.ActorOf<AllDeadLettersMessagesMonitorActor>();
+    
+    // Subscribe to messages of type AllDeadLetters
+    system.EventStream.Subscribe(monitorActor, typeof(AllDeadLetters));
+    
+    // try sending a message to actor which it doesn't understand
+    dumbActor.Tell("Hello");
+    
+    // simulate the actor failing/stopping
+    dumbActor.Tell(Akka.Actor.PoisonPill.Instance);
+    
+    // try sending a message to the stopped actor
+    dumbActor.Tell("world");
+    
+    Console.ReadLine();
+}
+```
+
+sample capture
+
+```string
+DeadLetter from [akka://MySystem/deadLetters] to [akka://MySystem/user/$a#1479030912]: <Hello>
+DeadLetter from [akka://MySystem/deadLetters] to [akka://MySystem/user/$a#1479030912]: <world>
+```
+
 ## Subscribing to Messages of Type `string`
 
 ```csharp